Conventional prospecting for oil and gas reservoirs is done by imparting acoustic energy into the earth, and detecting the imparted energy after reflection from or refraction through various formations. Analysis of certain attributes of the detected energy, such as travel time, provides an indication of the types and locations of sub-surface formations and interfaces encountered by the energy along its source-receiver path. As a result, a survey of the sub-surface geology can be generated for the region of interest, conventionally in the form of a contour map indicating the location, depth and acoustic velocity of various sub-surface strata. From such maps and surveys, skilled geologists and geophysicists can infer the location and depth of potential hydrocarbon reservoirs.
While conventional analysis of such seismic survey data is successful to a large degree, certain inaccuracies often exist. For example, noise which is picked up by the receivers may mask certain energy, limiting the ability to detect a particular sub-surface interface. Poorly designed surveys can result in spatial aliasing and related undesirable effects. Furthermore, inaccuracies in the estimated acoustic velocity of particular formations, or in other assumptions used in data interpretation, will be manifest as inaccuracies in the resulting survey or contour map. It is not uncommon for errors in the position or depth (or both) of a particular geological structure to have magnitudes in the hundreds of feet.
Due to the likelihood of such errors, a method of verifying the accuracy of a survey would be desirable, at a minimum; a method of actually correcting or adjusting the results of a survey would be especially desirable. Prior to the present invention, the use of models, either numerical or physical, of regions of the earth have been considered to be useful for such verification. Construction of a model of the surveyed region, as indicated by the contour map, followed by the performing of a "seismic" survey on the model, allows for a comparison of the seismic data acquired from the model with that from which the contour map was constructed. Differences in the field seismic data from the model would indicate inaccuracies in the seismic data interpretation process. Re-interpretation of the previously acquired seismic data, or even acquisition of new seismic data from the region of interest, could then be performed to provide a new or adjusted survey of sufficient accuracy that drilling could be performed with reasonable confidence.
Conventional techniques for creating scale models of the earth include the creation of plaster and wood molds to match the shapes in the contour map. The material molded by such molds is generally one of several two-part rubber or plastic materials having the desired physical properties, such as acoustic velocity (compressional, horizontal and vertical shear), density, and other elastic material properties. The layer becomes a mold for the next layer that bonds thereto, with the result being a laminated block of dimensions on the order of one to three feet on a side to represent the surveyed region. Scaled acoustic or ultrasonic sources and detectors are then deployed at the surface of the model, generally near the center of the top surface so that boundary effects at the sides of the model are effectively infinitely distant, and a scaled seismic survey is performed to simulate an actual field survey.
However, these prior scale models have not provided sufficiently accurate information on a timely enough basis to allow for useful verification of the survey information. This is due to the time-consuming and expensive construction of the plaster and wooden molds, which preclude testing in a sufficiently timely fashion to meet business needs. In addition, the imprecision of this fabrication technique adds dimensional inaccuracy in the structure of on the order of tenths of inches; for a typical scale of 1 inch to 1000 feet, such inaccuracy corresponds to hundreds of feet in the surveyed region. As a result, not only is the simulation late, the results are also inaccurate to such an extent that one cannot determine if a deviation in the data is due to inaccuracy in the scale model, or truly due to inaccurate interpretation of the field data.
Of course, actual measurement of the finished scale model would allow accounting for dimensional inaccuracy of the model in analyzing the simulation data. However, actual measurement of the model must be performed in a non-contact manner so that the model can be useful after such measurement. Conventional non-contact measurement techniques, such as x-ray, CAT scan, or another imaging technique, are not only expensive, but are quite cumbersome for objects of the size of these models.
These prior models also have the distinct disadvantage of being single-use structures. This is due primarily to the imprecision of the molds used to form the model structure, which requires the layers in these models to be bonded together to form a single laminated structure. If simulation of the survey of a similar region of the earth is desired over varying physical properties for a given layer, each simulation iteration requires a completely new structural model. However, the presently-achievable accuracy in building such conventional models makes it impossible to repeat a model experiment with a new model structure which has only one feature changed. In addition, as noted above, the time and expense of constructing such a new model structure also prohibits such iterative analysis.
For the above-stated reasons, as well as others, little use is currently made of scale models for verification and correction of seismic surveys.
By way of further background, recent advances have been made in the field of the fabrication of parts from computer-aided design (CAD) data bases. A summary of these recent advances and techniques has been recently published in Wood, "Desktop Prototyping", Byte (May 1991), pp. 137-142, incorporated herein by reference.
An apparently useful method for the formation of prototype parts, noted in the Wood article, is commonly referred to as stereolithography. According to this method, the surface of a vat of curable resin is irradiated by a laser according to a cross-sectional layer of the part to be produced. The resin polymerizes, or cures, at the irradiated locations, forming a solid mass thereat. The mass is slightly lowered into the vat, and the surface is again irradiated according to the shape of the next cross-section of the part, again forming a polymerized mass at the surface which adheres to the preceding layer. The process continues in layerwise fashion, until the part is completed and at which time the uncured liquid is removed. This process is more completely described in U.S. Pat. No. 4,575,330, issued Mar. 11, 1986 and incorporated herein by this reference. Various models of apparatus for performing this process are now available from 3D Systems, Inc., such models including the SLA-190, SLA-250 and SLA500.
Other additive methods for producing parts from computer aided data bases are also now becoming available, as indicated in the Wood article noted hereinabove.
It is an object of this invention to provide a method of rapidly fabricating accurate scale models of the earth, so that seismic surveys may be timely verified and corrected.
It is a further object of this invention to provide a method of forming such models which allows for development of survey and data interpretation techniques.
It is a further object of this invention to provide a modular scale model so that efficient and inexpensive iteration of particular attributes of the model can be performed.
It is a further object of this invention to provide such a method which utilizes computer-aided design tools so that the construction of various structures can be readily implemented.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification.